The economical packaging of RF systems and modules is complicated by the need to prevent leakage of RF signals from one part of the system to another, or from the system to the outside environment. The problem is compounded by the need for systems to be easily assembled and maintained. It is a particular problem with amplifier modules, in which high-amplitude signals, low-amplitude signals, and high-gain circuit components must be packaged together in a small volume.
To achieve low RF leakage between parts of a subsystem, an RF subsystem is typically partitioned into a number of separate physical compartments. Interconnection of the compartments often requires additional manufacturing steps including hand assembly, which significantly increases costs. The separate physical compartments may also complicate testing and adjustment of the subsystem.
A feed-forward amplifier module, such as that of the present invention, presents a particularly difficult packaging problem, since within a small volume several separate amplifiers having high gains must be packaged.
Error correction of amplifiers grows ever more important as frequency allocation, frequency reuse, and RF interference (RFI) become more prevalent. In the amplification of electronic message signals one paramount consideration is to maintain signal fidelity, while another paramount consideration is to minimize the operating power requirements. Those two factors are in conflict with each other. High efficiency amplifiers will typically have high signal distortion, but high-fidelity amplifiers typically require high operating power.
One approach to obtaining both high efficiency and low distortion is the use of cascaded feed-forward amplifiers as discussed in copending application Ser. No. 09/083,579. The feed forward amplifier system utilizes a main amplifier having a high power efficiency, and which is permitted to operate with high signal distortion. Associated circuits then observe and measure the distortion or error, and produce a correction signal which is added into the final output of the amplifier system so as to offset or counteract the signal distortion. A comparison is made between input and output signals of the main amplifier in order to provide an error signal, and a separate error amplifier is utilized to amplify the error signal before its re-insertion with appropriate polarity into the main amplifier path.
Feed forward amplifier systems typically have poor power efficiency, however. The efficiency can be greatly improved by making the error amplifier itself a feed-forward amplifier. While improving system efficiency, this comes at the cost of a much more complex error amplifier, making efficient and effective packaging of the error amplifier critical if the system is to be commercially competitive.